This invention relates to automatic gain control loops and, more particularly, to an automatic gain control loop that uses a non-linear reference to linearize the closed loop system.
There are various techniques for linearizing the response of an automatic gain control loop to control an amplifier""s output power. One technique employs a logarithmic amplifier positioned after a power detector, which detects exponential response of the power amplifier, in order to linearize the overall loop response; the logarithm of an exponential response yields a linear transfer function. Another technique includes an xe2x80x9cinverse plant blockxe2x80x9d for compensation. An xe2x80x9cinverse plant blockxe2x80x9d takes the non-linear transfer function of the closed loop and maps it to a circuit which will duplicate its exact inverse response. Other techniques make use of variable attenuators which have linear control in terms of dBs of attenuation, allowing for a linear control loop to be developed. Still other techniques take advantage of a linear xe2x80x9creceived signal strength indicatorxe2x80x9d (RSSI) for detection which can provide a linear transfer function in terms of Volts/dBm.
Each of the above techniques requires linearization of the output of the power detector or other types of additional circuitry which add substantial cost to the linear power control loop. Further, each of the above techniques is likely subject to significant changes in expected output due to temperature variations to which the linear power control loop may be subjected.
Thus, in view of the above, there is a need for a linear power control loop which does not require linearization of the output of the power detector, which does not require substantial amounts of additional circuitry, and which can maintain a substantially reliable linear output over a wide range temperature variations while providing a low cost to the user.
The needs described above arc in large measure met by a linear power control loop of the present invention. Specifically, the present invention presents a closed loop system that utilizes a non-linear reference to control a power amplifier""s output power in order to obtain a linear transfer function of dB per adjustment step of a reference input. The closed loop system demonstrates that each non-linear stage/step in an automatic gain control system can create a linear closed loop system when using a non-linear reference. The closed loop system of the present invention eliminates the need for a linearization circuit for the system""s power detector. The closed loop system may be used with most power amplifiers when linear control in terms of dB vs. adjustment setting of the input reference signal is desired. Output power in terms of dBms can be accurately set in linear steps where power control over a wide dynamic range is desired.
The linear power control loop generally includes a power amplifier, a power detector, an adjustable, non-linear reference signal, and a comparator. The power amplifier is provided with a power input signal and a control input to which, in response thereto, produces a substantially linear, transfer function due to feedback control from the loop. The power amplifier on its own is a non-linear device whose output power, in dBm, responds non-linearly to an input control voltage. The power detector operates to determine the magnitude of the output power of the power amplifier and to produce a voltage output. This voltage output, which is generally non-linear in nature but proportional to the input power, is compared, by virtue of the comparator, with the adjustable, non-linear reference signal. The output of the comparator represents the difference between the power detector output and the non-linear reference signal. The output of the comparator is provided to the power amplifier in the form of the control input voltage. Each adjustment in the non-linear reference signal produces a variation in the power output of the loop; the power output with respect to the reference signal, i.e., the closed loop transfer function, is linear. The adjustments made to the reference signal are preferably made in linear steps.
The adjustable, non-linear reference signal is preferably provided by a programmable potentiometer, e.g., EEPOT. As stated earlier, this non-linear reference signal is compared with the power detector""s voltage output. The power detector output is provided directly to the comparator from the power detector absent any intermediate circuitry such as linearization circuits that have been used in prior art circuits, which would tend to add cost to the control loop. It should be noted that the power detector may be a temperature compensated power detector adjusting for variations in circuit operation due to changes in temperature. Further, it should be noted that the comparator preferably incorporates a filter to filter, the comparator output to provide a stable output signal and to set the loop bandwidth. The linear power output control loop is able to provide a substantially linear output in terms of dB per linear adjustment of the reference signal, due to the logarithmic nature of the reference signal.
A method for controlling a power amplifier to produce a substantially linear power output in dBs generally includes the following steps: (1) providing a power input to the power amplifier; (2) producing a power output from the power amplifier; (3) detecting the power output; (4) providing an adjustable, non-linear reference signal; (5) comparing the adjustable, non-linear reference signal voltage with the detected power output voltage; (6) producing an error output that is representative of the difference between the non-linear reference signal and the detected power output voltage; (7) providing the error voltage to the power amplifier in the form of a control input; and (8) amplifying the power input with the power amplifier with a suitable gain in response to the control input in order to achieve the desired output power, whereby the output power is linear with respect to each adjustment in the non-linear reference signal. Of course, the above-mentioned steps may be performed in any appropriate order.